DESCRIPTION

[0001] The present invention relates to a spark ignition engine and a fueling method of such an engine.

[0002] The increasing concerns about fuel depletion and environmental matters have caused a search for new technologies for increasing fuel saving and reducing C02 emissions and polluting emissions, while at the same time safeguarding engine performance. Automobile manufacturers are leaning towards direct injection also for spark ignition engines. Engines with GDI (Gasoline Direct Injection) systems allow an increased efficiency and an increased level of power to be obtained with respect to PFI (Port Fuel Injection) systems. The injection of the fuel directly into the cylinder increases the antiknock power and the volumetric efficiency due to the improved cooling of the air due to the evaporation of the fuel. The quantity of fuel may be controlled in a more accurate manner with subsequent improved engine operation under partial load conditions and during dynamic operation. The main drawback of GDI technology is the increase of HC, CO emissions and particles due to the short time available for the evaporation of the fuel and for the formation of the mixture, and also due to the knocking of the fuel on the piston head and on the cylinder wall. A stratified lean burn may reduce the emissions of unburnt hydrocarbons in the cold start and the CO2 emissions, but the engine stability worsens. The CCVs (Cycle to Cycle Variations) play an important role for engine performance. Indeed, the operating instabilities cause engine vibrations and noise, which reduce the power delivered. On the other hand, the reduction of the CCV causes an increase of power delivered, fuel consumption being equal. Various sources act on the CCV in a spark ignition engine. In particular, the intensity of the turbulence of the flow field in the cylinder, the variations in the air-fuel ratio, the quantity of residual exhaust gases or exhaust gasses recirculated in the cylinder, the spatial inhomogeneity of the composition of the mixture, especially close to the spark plug, the spark discharge features and the propagation of the flame front play an important role in the worsening of the CCV. The increase of the combustion speed translates into a reduction of the IMEP CoV (Indicated Mean Effective Pressure Coefficient of Variation).

[0003] It is a general object of the present invention to make available a spark ignition engine which allows the above-mentioned drawbacks of the art disclosed with reference to spark ignition engines with GDI systems of the known art, to be resolved or at least partly obviated.

[0004] It is a particular object of the present invention to make available a spark ignition engine characterized by an improved operating stability and a reduced specific consumption of fuel with respect to the above-described spark ignition engines with GDI systems of the known art.

[0005] These and other objects are achieved by means of a spark ignition engine as defined in claim 1 in the most general embodiment thereof, and in the claims dependent thereon in certain particular embodiments thereof.

[0006] The object of the present invention is also a fueling method of a spark ignition engine as defined in claim 8.

[0007] A more complete appreciation of the embodiments of the invention and the related associated advantages will be apparent from the following detailed disclosure, to be considered in connection with the accompanying drawings, in which:

Figure 1 shows an axonometric sectional view of a part of a spark ignition engine according to one embodiment of the present invention;

Figure 2 shows an almost flat axonometric bottom view of the cylinder head of the engine in figure 1 ;

Figure 3 shows a view diagrammatically showing certain geometric features related to the injection of the gasoline into the engine in figure 1 ;

Figure 4 shows a view showing a graph diagrammatically depicting the trend of the pressure in a combustion chamber of the engine in figure 1 according to the crank angle in two successive cycles of the engine.

[0008] The following description of one or more embodiments of the invention relates to the accompanying drawings. The same numerals in the drawings identify equal or similar elements. The drawings are not necessarily depicted to scale. Moreover, the following detailed description should not be considered limiting to the invention itself. The object of the invention is defined by the appended claims. The technical details, structures or features of the solutions hereinafter described may be combined with one another in any manner, unless it is expressly indicated that certain features or solutions are alternative to one another or it is apparent from the description that follows that two or more feature or embodiments are incompatible with one another.

[0009] Unless otherwise indicated, the term“radial” or similar terms in the following description refer to the central axis X1 of cylinder 10 (axis X1 and cylinder 10 are described further below). In other words, the term“radial” refers to a circumference lying on a plane orthogonal to axis X1 and having the center on such an axis X1.

[0010] With reference initially to figure 1 , a spark ignition engine according to a preferred embodiment is generally indicated with numeral 1. Engine 1 comprises a cylinder 10 having a cylinder central axis X1 , a cylinder head 1 1 and a piston 20 movable along the central axis X1. The cylinder head 1 1 has at least one intake valve 12 and at least one discharge valve 13, and even more preferably two intake valves 12 and two discharge valves 13. The valves 12, 13 preferably are poppet valves. A combustion chamber 30 is adapted to be defined between the cylinder head 11 and piston 20. Engine 1 comprises a PFI injector 40 arranged to inject a fuel 41 in PFI mode. In other words, the injector 40 is arranged so as to inject fuel 41 into a supply duct 42, or intake duct 42, upstream of cylinder 10. According to one embodiment, injector 40 only injects fuel 41 , preferably in the form of a spray.

[0011] According to an embodiment, engine 1 further comprises at least one gas Dl (Direct Injection) opening 51 arranged to inject a gas 52 in Dl mode. In other words, the at least one opening 51 is configured to inject gas 52 directly into the combustion chamber 30. According to a preferred embodiment, the gas 52 injected through the at least one opening 51 is air 52. Flowever, as is better understood in the following, gas 52 generally may comprise any non-combustible or substantially non-combustible gas, such as for example, the exhaust gas generated following the combustion which occurs in cylinder 10 during the operation of engine 1. Thus, it is understood that when reference is made, in the following, to air 52 to describe the structure and/or the operation of engine 1 and parts thereof, similar considerations are in general also valid for a gas 52 other than air.

In the example, the at least one opening 51 is at least one air Dl opening which is arranged to inject air 52 in Dl mode. Reference is made in the following to opening 51 only as an air Dl opening, but as will be more apparent below, it is understood that generally a gas other than air can also be injected through opening 51. Engine 1 comprises only one opening 51 in the non-limiting example. However, according to an alternative embodiment, engine 1 may comprise a plurality of openings 51. According to one embodiment, fuel 41 is gasoline 41. In the example, engine 1 is a four-stroke single-cylinder engine having a capacity of 250cm ³ , a diameter of cylinder 10 equal to 72mm, a stroke of piston 20 equal to 60mm, a maximum torque equal to 20Nm at 5500 rpm, a maximum power equal to 16kW at 8000 rpm and a compression ratio of 1 1.5:1. However, it is worth noting that the teachings of the present description are not limited to such an engine, rather in general are applicable to any spark ignition engine, and in particular are also applicable to two-stroke engines and to engines having any number of cylinders. Conveniently, the air Dl opening 51 is configured to inject air 52 into the radial periphery (indicated by arrow F1 in figure 2), i.e. with a tangential flow which laps the combustion chamber 30. It is worth noting that according to a preferred embodiment, only air 52 is injected through opening 51.

[0012] In particular, the term “radial periphery” means an annular portion of the combustion chamber 30 located close to the radially inner side wall 10A of cylinder 10. Wall 10A is opposite to a radially outer side wall 10B of cylinder 10. The radial periphery F1 preferably has an outer diameter equal to the diameter of the side wall 10A of cylinder 10, an inner diameter equal to about half the diameter of wall 10A and a height equal to the distance between piston 20 and the cylinder head 1 1.

[0013] The injection into the radial periphery provides for the injected air flow to be substantially tangential to the combustion chamber. In other words, the flow defines a circular path so as to flow circularly over the periphery of the combustion chamber. In particular, the injected air flow causes a recirculation of air which flows circularly about the combustion chamber. The effect of the air flow is the creation a perimeter barrier to the load of fuel by concentrating it in the middle of the combustion chamber itself. [0014] It is also worth noting that injecting air 52 into the aforesaid radial periphery F1 allows such a stratification of the load, whereby air 52 injected through opening 51 concentrates and keeps the load of fuel 41 injected by means of injector 40 and of air coming from the supply duct 42 in the central part of the combustion chamber 30 close to the spark plug 60. In other words, the vortex of air that the injected air 52 creates in cylinder 10 concentrates the load of air and fuel 41 from the supply duct 42 during the compression stroke, towards the central axis X1. This advantageously allows both reducing the specific consumption of fuel and improving the stability of the operation of engine 1. Indeed, due to the stratification of the load obtained in the above-described manner, there is a reduction of the specific consumption with respect to a traditional GDI system due to:

- fewer losses due to heat exchanges with the walls of cylinder 10;

- reduced loss of negative work of the pumping cycle;

- higher compression ratio, which may be implemented due to the cooling of the load due to the injection of air, which allows increased performance to be obtained;

- possibility of using fuel with lower number of octanes;

- increased filling factor.

Moreover, an increased control and stability of the combustion is possible with regards to the operating stability due to the aforesaid load stratification because the impingement of the fuel on the cylinder walls is avoided, thereby allowing incorrect sparks of the fuel caused by problems of deposit formation on the cylinder walls, to be avoided.

[0015] According to a preferred embodiment, the at least one opening 51 is configured to inject air 52 towards piston 20. An injection axis P1 , corresponding to a prevalent direction of injection of air 52 injectable through the at least one opening 51 , is associated to the at least one opening 51. Moreover, a first direction Q1 is defined which lies on a plane K1 perpendicular to the central axis X1. The first direction Q1 connects the geometric barycenter 53 of the at least one air Dl opening 51 projected onto plane K1 and the cylinder central axis X1 . The geometric barycenter of the at least one opening 51 projected onto plane K1 practically corresponds to the geometric barycenter of the edge of the at least one opening 51 projected onto plane K1. It is worth noting that axis P1 is an axis passing through the geometric barycenter of the at least one opening 51. The edge of opening 51 typically is a closed line. For example, given that opening 51 preferably is a circular hole, the related edge preferably is a circular hole. In other words, if only one opening 51 is provided, the first direction Q1 connects the geometric barycenter 53 of the air Dl opening 51 projected onto plane K1 (i.e. the geometric barycenter of the edge of opening 51 projected onto plane K1 ) and the cylinder central axis X1. If instead a plurality of openings 51 is provided, direction Q1 connects the geometric barycenter of the various openings 51 (i.e. the geometric barycenter of the edges of openings 51 projected onto plane K1 ) and the cylinder central axis X1. It is worth noting that geometric barycenter of the edge of the at least one opening 51 projected onto plane K1 means, defined a system of Cartesian axes on plane K1 , a point which coordinates are given by the arithmetical average of the coordinates of each point of the edge of opening 51 projected onto plane K1. In addition to direction Q1 , a second direction Q2 also is defined which corresponds to the orthogonal projection of axis P1 onto the aforesaid plane K1. A first angle A1 is defined between the first direction Q1 and the second direction Q2. A second angle A2 is defined between the second direction Q2 and the axis P1 of opening 51. The first angle A1 is an angle between about 60° and about 90°. The second angle A2 is an angle between about 45° and about 90°.

[0016] According to a preferred embodiment, the air Dl opening 51 is arranged so as to inject air 52 tangentially to cylinder 10. In this regard, it is worth noting that the term“tangentially” means according to a direction which is tangent or substantially tangent to a circumference lying on plane K1 and having the respective center on axis X1. In other words, when air 52 is injected tangentially to cylinder 10, angle A1 is equal to 90° or about 90°, while angle A2 is equal to 0° or about 0°.

[0017] According to a preferred embodiment of engine 1 , the aforesaid spark plug 60 preferably is arranged centrally or substantially centrally with respect to the cylinder head 1 1.

[0018] According to a preferred embodiment, engine 1 comprises a Dl-gas injector 70 which comprises the at least one air Dl opening 51.

[0019] According to an alternative embodiment (not depicted), rather than providing the injector 70, the at least one air Dl opening 51 may be formed directly in the cylinder head 1 1. In this case, also a suitable air introduction duct preferably is provided inside the cylinder head 11.

[0020] With reference to figure 1 , according to one embodiment, engine 1 comprises a storage tank 80 for air 52 and a compressor 90 which is operatively connected to tank 80. Tank 80 and compressor 90 are diagrammatically depicted in figure 1 by means of rectangles. Compressor 90 is selectively operable to introduce the compressed air 52 into the storage tank 80. A valve 71 is interposed between tank 80 and opening 51. Valve 71 preferably is associated to injector 70. Valve 71 is controllable, preferably electronically controllable, so as to selectively place tank 80 in communication with opening 51 , and therefore with the combustion chamber 30. Practically, when air 52 is to be injected into the combustion chamber 30, valve 71 opens so that the compressed air stored in tank 80 may be introduced into the combustion chamber 30 through opening 51.

[0021] Also a fueling method of engine 1 practically was described on the basis of the above-described structure of engine 1. The fueling method comprises:

a) a step of injecting - preferably in the form of a spray - fuel 41 in PFI mode; and b) a step of injecting air 52 in Dl mode through at least one air Dl opening 51.

In step a), the injection of fuel 41 is performed in the supply duct 42 upstream of cylinder 10. In step b), the injection of air 52 is performed directly in the combustion chamber 30 of cylinder 10.

Step b) in particular comprises:

b1 ) an operation of injecting gas 52 into the radial periphery of the combustion chamber 30.

[0022] According to one embodiment, the aforesaid step a) comprises only injecting fuel 41 by means of injector 40. Clearly, fuel 41 injected by means of injector 40 is mixed with the intake air from the supply duct 42.

[0023] According to one embodiment, the aforesaid step b) comprises only injecting air 52. In other words, only air 52 is injected through opening 51 in this case.

[0024] According to one embodiment, the operation b1 ) comprises injecting the air through piston 20 according to direction P1 , in which the first angle A1 is an angle between about 60° and about 90°, and in which the second angle A2 is an angle between about 45° and about 90°. [0025] According to one embodiment, the operation b1 ) comprises injecting the air tangentially to cylinder 10. Said air 52 being injected during the compression stroke, i.e. when piston 20 moves towards the cylinder head 11.

[0026] According to one embodiment, the method comprises:

c) a step of ending the injection of air 52 of the aforesaid step b) prior to the production of a spark by the spark plug 60.

[0027] According to one embodiment, step c) comprises ending the injection of air 52 at the latest 1 ms prior to the production of the spark by the spark plug 60.

[0028] According to one embodiment, step b) comprises injecting air 52 at an injection pressure Pi which is equal to or greater than 3.5 bar.

[0029] According to one embodiment, air 52 injected in step b) is about 2% to 3% in volume of the air sucked (under standard operating conditions) through the intake duct 42 of engine 1.

[0030] With reference to figure 4, such a figure shows a graph diagrammatically showing the trend of pressure Pc in the combustion chamber 30 as the crank angle CAD varies during two cycles C1 , C2 of engine 1 immediately following each other, where the term“cycle of the engine” means the group transformations undergone by the mixture of air and fuel inside cylinder 10 which is cyclically repeated during the operation of engine 1. In other words, a cycle of the engine consists of the steps of intake, compression, spark-expansion and discharge. The graph in figure 4 also indicates the injection pressure Pi of the air injected through opening 51 and the crank angles TDC corresponding to the top dead center in the cycles C1 and C2 of the engine.

[0031] Again with reference to figure 4, according to an advantageous embodiment, the fueling method comprises:

a1 ) a step of drawing exhaust gas 52 from the combustion chamber 30 during the expansion stroke EF1 relative to a first cycle C1 of the engine. In particular, such a step of drawing exhaust gas 52 starts when pressure Pc in the combustion chamber 30 reaches a drawing start predetermined pressure P1 -1 which is greater than the injection pressure Pi of the exhaust gas 52 to be injected through opening 51.

Moreover, following step a1 ), the fueling method comprises:

a2) a step of storing the exhaust gas 52, which has been drawn in the aforesaid step a1 ), inside the storage tank 80 operatively connected to opening 51 until pressure Pc in the combustion chamber 30 reaches an end drawing predetermined pressure P1 -2 which is lower than the drawing start predetermined pressure P1 -1 and is greater than or equal to the injection pressure Pi.

Practically, according to one embodiment, when pressure Pc in the combustion chamber 30 reaches the drawing start pressure P1-1, valve 71 opens, thus allowing the communication between the combustion chamber 30 and the storage tank 80, and therefore the pressurization of tank 80 itself until the pressure Pc in the combustion chamber 30 reaches the end drawing pressure P1-2, which once reached, causes valve 71 to close, thus preventing the communication between the combustion chamber 30 and tank 80.

[0032] Following steps a1 ) and a2), the aforesaid step b) comprises injecting the exhaust gas 52 stored in the storage tank 80 during step a2) into the combustion chamber 30 during the compression stroke CF2 relative to a second cycle C2 of the engine following the first cycle C1 of the engine. In the example, the second cycle C2 of the engine is a cycle of the engine immediately following cycle C1 of the engine. Flowever, it is worth noting in general that the exhaust gas 52 stored in the storage tank 80 during step a2) may be injected into the combustion chamber 30 also after various cycles of the engine with respect to cycle C1. The aforesaid step b) starts when pressure Pc in the combustion chamber 30 reaches an injection start predetermined pressure P2-1 which is lower than the injection pressure Pi of the exhaust gas 52 and ends when pressure Pc in the combustion chamber 30 reaches an injection end predetermined pressure P2-2 which is greater than the injection start predetermined pressure P2-1 and lower than or equal to the injection pressure Pi of the exhaust gas 52.

Practically, according to one embodiment, when pressure Pc in the combustion chamber 30 reaches the injection start pressure P2-1 in the second cycle C2 of the engine, valve 71 opens, thus allowing the communication between the combustion chamber 30 and the storage tank 80, and therefore the injection of the exhaust gas 52 into the combustion chamber 30 through opening 51 until the injection end pressure P2-2 is reached. Once such a pressure P2-2 is reached, valve 71 closes, thus preventing the communication between the combustion chamber 30 and tank 80. [0033] An alternative embodiment of the above-described fueling method is now described. Such an alternative embodiment can be implemented in particular if engine 1 is a multi-cylinder engine. The same numerals used above to describe the various elements of engine 1 are used in the description below to describe corresponding elements of the multi-cylinder engine. In particular, the various cylinders of such an engine are indicated with the same numeral 10. It is also worth noting that the cycles C1 and C2 in figure 4 in the following description of the alternative embodiment of the fueling method refer to two different cylinders 10 of engine 1 rather than to a same cylinder of the engine.

[0034] Such an alternative embodiment of the fueling method comprises, alternatively to step a1 ):

- a1 ^* ) a step of drawing exhaust gas 52 from the combustion chamber 30 of a first cylinder 10 of engine 1 during the expansion stroke EF1 relative to a first cycle C1 of the engine associated to the first cylinder 10. In particular, such a step of drawing exhaust gas 52 starts when pressure Pc in the combustion chamber 30 of the first cylinder 10 reaches a drawing start predetermined pressure P1 -1 which is greater than the injection pressure Pi of the exhaust gas 52 to be injected through the at least one opening 51 associated to a second cylinder 10 of the engine.

Moreover, following step a1 ^* ), the fueling method comprises, alternatively to step a2):

- a2 ^* ) a step of storing the exhaust gas 52 drawn in the aforesaid step a1 ^* ) inside the storage tank 80 operatively connected to the at least one opening 51 of the first cylinder 10 until pressure Pc in the combustion chamber 30 of the first cylinder 10 reaches an end drawing predetermined pressure P1 -2 which is lower than the drawing start predetermined pressure P1 -1 and is greater than or equal to the injection pressure Pi of the exhaust gas 52 to be injected through the at least one opening 51 associated to the second cylinder 10 of the engine.

According to the aforesaid alternative embodiment of the fueling method, step b) starts when pressure Pc in the combustion chamber 30 of the second cylinder 10 reaches an injection start predetermined pressure P2-1 which is lower than the injection pressure Pi of the exhaust gas 52 to be injected through the at least one opening 51 associated to the second cylinder 10, and ends when pressure Pc in the combustion chamber 30 of the second cylinder 10 reaches an injection end predetermined pressure P2-2 which is greater than the injection start predetermined pressure P2-1 and lower than or equal to the injection pressure Pi of the exhaust gas 52 to be injected through the at least one opening 51 associated to the second cylinder 10.

Practically, according to one embodiment, when pressure Pc in the combustion chamber 30 of the first cylinder 10 reaches the drawing start pressure P1-1, valve 71 associated to the first cylinder 10 opens, thus allowing the communication between the combustion chamber 30 and the storage tank 80, and therefore the pressurization of tank 80 itself until pressure Pc in the combustion chamber 30 of the first cylinder 10 reaches the end drawing pressure P1-2, which when reached, causes valve 71 associated to the first cylinder 10 to close, thus preventing the communication between the combustion chamber 30 and tank 80.

[0035] Following steps a1 ^* ) and a2 ^* ), the aforesaid step b) comprises injecting the exhaust gas 52 stored in the storage tank 80 during step a2 ^* ) into the combustion chamber 30 of the second cylinder 10 of the engine during the compression stroke CF2 relative to a second cycle C2 of the engine associated to the second cylinder 10 and following the first cycle C1 of the engine associated to the first cylinder 10. Clearly, both the first cylinder 10 and the second cylinder 10 of the engine are operatively connected to the storage tank 80.

[0036] Practically, according to one embodiment, when pressure Pc in the combustion chamber 30 of the second cylinder 10 reaches the injection start pressure P2-1 in the second cycle C2 of the engine of the second cylinder 10, valve 71 associated to the second cylinder 10 opens, thus allowing the communication between the combustion chamber 30 of the second cylinder 10 and the storage tank 80, and therefore the injection of the exhaust gas 52 into the combustion chamber 30 of the second cylinder 10 through opening 51 associated to the second cylinder 10 until the injection end pressure P2-2 is reached. Once such a pressure P2-2 is reached, valve 71 associated with the second cylinder 10 closes, thus preventing the communication between the combustion chamber 30 of the second cylinder 10 and tank 80.

[0037] In other words, the substantial difference between the embodiment of the fueling method comprising steps a1 ), a2), b) and the embodiment comprising steps a1 ^* ), a2 ^* ), b) is the fact that the exhaust gas in the first case is drawn from a cylinder and reinjected into the same cylinder, while the exhaust gas in the second case is drawn from a first cylinder and reinjected into a second cylinder of the engine which is conveniently phased with the first cylinder.

[0038] The above-described embodiments of the fueling method comprising steps a1 ), a2), b) and steps a1 ^* ), a2 ^* ), b), respectively, advantageously allow avoiding the use of compressor 90 to compress gas 52 to be injected into the combustion chamber 30, and accordingly obtaining a simpler and more compact system.

[0039] It is also worth noting that the above-described embodiments of the fueling method comprising steps a1 ), a2), b) and steps a1 ^* ), a2 ^* ), b), respectively, advantageously allow the local production of NOx to be reduced with respect to a traditional GDI system, due to the fact of allowing the recirculation of the exhaust gas.

[0040] According to that described above, it may be understood how a spark ignition engine and a fueling method of a spark ignition engine according to the present description allow the above-mentioned objects with reference to the known art to be achieved.

[0041] In conclusion, while the embodiments of the inventive object herein described are shown in the drawings and described in detail above, it is apparent to those skilled in the art that the invention thus conceived is susceptible to several modifications or variants, all falling within the invention; moreover, all the details may be replaced by technically equivalent elements. Practically, the quantities may be varied according to technical needs. Therefore, the object of the innovation herein disclosed is to be considered broader in the case at hand than the accompanying claims so as to include all possibilities of modification, replacement or omission.